The present invention relates generally to ion implanters, and more specifically to an in-process wafer charge monitor and control system for such ion implanters.
Conventional ion implantation systems, used for doping workpieces such as semiconductors, typically include an ion source that ionizes a desired dopant element which is then accelerated to form an ion beam of prescribed energy. The ion beam is directed at the surface of the workpiece to implant the workpiece with the dopant element. The energetic (generally positive) ions of the ion beam penetrate the surface of the workpiece so that they are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity. The implantation process is typically performed in a high-vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particulates.
A problem encountered in the use of such an ion implantation system is that of wafer charging. As the positively charged ion beam continues to impinge upon the target wafer, the surface of the wafer may accumulate an undesirable excessive residual positive charge. In the case of wafers covered by an insulating material such as photoresist, the wafer charging phenomenon is particularly problematic because the charge is isolated from the semiconductive wafer substrate and the pedestal upon which is resides, and cannot therefore be dissipated through the wafer and/or the wafer pedestal.
Excessive charge accumulation can cause resulting electric fields at the wafer surface that can damage microcircuitry on the wafer. The problem of accumulated surface charge becomes more pronounced as implanted circuit elements become smaller, because smaller circuit elements are more susceptible to damage caused by the resultant electric fields.
A known solution to the wafer charging phenomenon is the use of a charge neutralization, or charge control, system. Such a system typically includes a plasma shower that provides a source of low energy electrons that are drawn into the positively charged ion beam passing thereby. Specifically, the plasma shower includes an arc chamber in which an inert gas is ionized to produce a plasma comprised at least partially of low energy electrons, and a plasma chamber into which the plasma is extracted from the arc chamber and through which the ion beam passes. The plasma contains a filament that is electrically heated so that it thermionically emits high energy electrons into the plasma chamber. The high energy electrons collide with the inert gas molecules to create the plasma which includes low energy electrons capable of being trapped within the ion beam. The trapped low energy electrons neutralize the net charge of the ion beam and are transported to the wafer surface by the ion beam. The trapped low energy electrons in the ion beam reduce or neutralize the positive charge accumulation on the wafer surface, caused by the implantation of positive ions, as the ion beam strikes the wafer surface.
Such charge neutralization systems or plasma showers typically include a charge neutralization monitor for monitoring the charge neutralization system to help control the charge neutralization process. Such a system is shown in U.S. Pat. No. 5,959,305 to Mack et al., which discloses a charge neutralization monitor that (i) applies a suitable voltage to a target electrode positioned to collect low energy neutralizing electrons and (ii) determines the available low energy neutralizing electron current that may be produced by the charge neutralization system by monitoring the current flowing through the target electrode.
It is difficult to determine the effectiveness of known charge control systems while ion implantation is occurring. If the charge control system is not providing adequate neutralization of the wafer surface, the residual charge accumulation generates a measurable voltage on the surface of the wafer that has at least two detrimental effects. First, the voltage differential between the implanted wafer surface and the wafer backside may rise to a level that will damage devices being fabricated on the wafer.
Second, the voltage present on the surface of the wafer can cause the ion beam to change shape as it scans from wafer to wafer. For example, wafers in some ion implantation systems typically reside upon wafer pedestals that populate the periphery of a spinning conductive (e.g., aluminum) disk. As the disk spins, the fixed-position ion beam passes from portions of the conductive aluminum disk surface intermediate wafers, to the insulative charged surface of a particular wafer being implanted, and back across conductive disk portions before reaching an adjacent wafer. The voltage present on the surface of a wafer can cause the ion beam to change shape as it passes from one of these surfaces to the next. As such, the voltage on the wafer can cause a non-uniform implanted dose across the planar surface of the wafer, resulting in the well-known xe2x80x9cbull""s-eyexe2x80x9d pattern of implant dosage.
Most ion implanter charge control systems utilize one or more charge monitor pick-ups that attempt to estimate the voltage levels present on the surface of a wafer being implanted. Such charge monitor pick-ups are often referred to as xe2x80x9cdisk Faradaysxe2x80x9d. The reading provided by the charge pick-up monitor(s) can be used to predict whether devices under fabrication are in danger of being damaged by excessive charge accumulation. Examples of such charge pick-up monitors are shown in U.S. Pat. No. 5,998,798 to Halling, et al., which is hereby incorporated by reference as if fully set forth herein. However, no known charge monitors in ion implantation charge control systems, including that disclosed in U.S. Pat. No. 5,998,798, currently provide any evidence of changes in beam shape as the beam passes between conductive portions of the spinning disk and the insulating portions of the wafers residing thereon.
It is an object of the present invention, then, to provide a mechanism by which changes in ion beam shape may be determined and accounted for in an ion implantation system. It is a further object to provide a mechanism by which one can prevent non-uniform implanted dose across the planar surface of the wafer such that the implanted wafer does not exhibit the well-known xe2x80x9cbull""s-eyexe2x80x9d pattern of implant dosage. It is yet a further object to provide in improved in-process charge monitor and control system for an ion implanter, wherein the effectiveness of a charge neutralization mechanism may be verified in real time and adjusted if necessary.
An in-process charge monitor and control system for an ion implanter is provided. The monitor and control system includes a rotating wafer support upon which a plurality of wafers may be positioned for implantation by an ion beam, the support having portions thereof disposed intermediate adjacent wafers that are more or less electrically conductive than surfaces of the wafers. Each of the plurality of wafers is positioned substantially equidistant from the center of the disk. The disk is also provided with first and second apertures disposed substantially equidistant from the center, wherein the first aperture located closer in proximity to a wafer than the second aperture. Alternatively, the first and second apertures may each be located equidistant from a wafer but surrounded by portions of the disk having different electrical conductivity characteristics. For example, the first aperture may be provided in a portion of the disk that is aluminum, and the second aperture may be provided in a portion that is silicon coated.
First and second electrical charge monitors positioned behind the disk receive first and second portions of the ion beam through the first and second apertures, respectively. The first and second charge monitors output first and second output signals, respectively, indicative of an amount of ion beam current received by the monitor. A comparator compares the first and second output signals and outputs a third output signal indicative of a difference in ion beam current received by first and second electrical charge monitors. The third output signal is used as an input to a charge neutralization system of the ion implanter to control the supply of low energy electrons produced thereby.